1. Field of the Invention
The present invention relates to solid-state imaging devices. In particular, the present invention relates to a solid-state imaging device that generates minimal random noise and can pick up moving images with high definition.
2. Description of the Related Art
With development in information technology (IT), the demand for area sensors has been rapidly increasing in various applications. In particular, CMOS sensors, which need less electricity to work as compared with CCD sensors, attract attention in applications such as digital cameras and portable terminals. Furthermore, with the increasing demand for higher image quality, noise reduction in area sensors is needed.
The mechanism that causes generation of random noise in a known CMOS sensor will be described with reference to FIG. 5. The CMOS sensor is formed on an n-type semiconductor substrate 401 and includes a p well layer (PWL) 402, a LOCOS oxide film 403, and a gate oxide film 404. The CMOS sensor further includes a photodiode composed of a p+ region 405 and an n region 406. The n region 406 is preliminarily depleted. Light incident on the n region 406 generates charges that are accumulated therein. In a readout process, a positive potential is applied to a gate electrode 407 to switch on the transfer transistor so that the charges are transfer to a readout region 408 formed of a high n region. The threshold of the transfer transistor is controlled by an n region 409.
A CMOS sensor having a structure such as that shown in FIG. 5 has the following problems. Upon occurrence of the charge transfer in the CMOS sensor, a channel is generated in the gate electrode 407 at a superficial position near the gate oxide film 404. In the photodiode, the p+ region 405 suppresses the effect of the surface defect and the n region 406 lies at a deep position a long way from the gate oxide film 404. Upon occurrence of the charge transfer, a potential barrier, which is formed at a connection 410 between the deep position and the superficial position, inhibits the charge transfer. Thus, the charges partially remain in the photodiode. When the number of charges remaining in the photodiode is n, the read data includes a variation of, which is a random noise of the sensor.
In a method designed to prevent the random noise of the sensor, the n region 406 is made to extend under the gate electrode 407 so that the n region 406 and an nxe2x88x92 region 409 overlap with each other. In this case, however, the connection 410 has a low-potential region at the overlap position. When the gate electrode is switched off after the charge transfer, the charges formed in the channel remain in the low-potential region and behind the n region 406, also causing the generation of random noise.
It is an object of the present invention to provide a solid-state imaging device having a structure capable of complete charge transfer to prevent charges from remaining in the photodiode after the readout process. The solid-state imaging device generates minimal random noise, can be used for high-speed readout, and can pick up moving images with high definition.
It is another object of the present invention to provide a method for driving the solid-state imaging device.
According to the present invention, a solid-state imaging device includes a photodiode including a well region of a first conductivity type, a first impurity region of a second conductivity type, and a heavily doped impurity region of a first conductivity type formed on the first impurity region; a gate electrode that transfers charges accumulated in the photodiode; a readout region comprising an impurity region of a second conductivity type; a second impurity region of a second conductivity type extending from the first impurity region to the readout region; and an impurity region of a first conductivity type formed below the gate electrode.
When a transfer transistor in the solid-state imaging device is switched on in this configuration, the charges flow in a region below gate electrode, i.e. the impurity region of a first conductivity type. Thus, no potential barrier is generated at a connection to the first impurity region of a second conductivity type, resulting in complete transfer of the charges accumulated in the first impurity region of a second conductivity type. As a result, no residual charge is present after a readout operation, reducing random noise of the solid-state imaging device.
In the solid-state imaging device, the second impurity region preferably has an impurity concentration such that the second impurity region is completely depleted when charges are accumulated. Preferably, the second impurity region is completely depleted below the gate electrode.
Since no carrier is present when the second impurity region of a second conductivity type is completely depleted, no current flows over the first impurity region of a second conductivity type and the readout region. Thus, charges can be effectively accumulated.
In the solid-state imaging device, a potential that is lower than that applied to the well region is preferably applied to the gate electrode when charges are accumulated.
In this configuration, the electric field effect of the gate electrode facilitates the expansion of the depletion layer in the second impurity region and increases the potential of the second impurity region below the gate electrode, resulting in accumulation of larger amounts of charges in the photodiode.
In the solid-state imaging device, the depth of the second impurity region may be smaller than the depth of the first impurity region.
In the solid-state imaging device, preferably a part of the first impurity region overlaps with the second impurity region at a position in which the first impurity region is in contact with the gate electrode, and another part of the first impurity region does not overlap with the second impurity region.
In this configuration, charges generated in a high depth position in the photodiode are accumulated in a lower-potential position near the surface. This effect enhances the possibility of capturing the charges generated in a large depth side in the photodiode, enhancing the sensor sensitivity. Furthermore, the charges are accumulated near a channel, which is generated in the ON mode of the transfer transistor, resulting in high-speed operation due to an increased transfer rate.
In the solid-state imaging device, the second impurity region may be formed by self-alignment using a device-isolating film by ion implantation.
In the solid-state imaging device, the second impurity region may be formed by ion implantation by resist patterning.
In the solid-state imaging device, the impurity region of a first conductivity type below the gate electrode may be formed by self-alignment using a device-isolating film by ion implantation.
In the above configurations, only the ion-implanted region of the second impurity region is ion-implanted by resist patterning. Thus, the concentration near the gate electrode is higher than that further away from the gate electrode. Thus, the potential is low near the gate electrode, so that the charges are concentrated to the channel region below the gate electrode by diffusion. This phenomenon increases the charge transfer rate, resulting in high-speed readout. Thus, this solid-state imaging device is suitable for a high-definition area sensor and can pick up moving images with high definition.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).